1. Field of the Invention
The present invention relates to a device manufacturing method using a lithographic apparatus and a mask for use in the method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
A conventional binary mask for use with DUV exposure radiation comprises a quartz substrate with a pattern defined thereon in a thin, normally 50 to 100 nm thick, absorber layer of chrome or chrome oxide. Although the chrome layer is essentially opaque, the contrast of the image at substrate level is not particularly high, especially at high NA values and small critical dimension. A low contrast reduces the useable process window since the dose and exposure duration must be carefully controlled to ensure the dark areas are below, and the light areas above, the resist threshold. The requirements on the resist threshold also become more strict.